Since the energy density available from batteries based on oxidation/reduction reactions of ions in the electrolyte solution is directly proportional to the concentration of redox ions undergoing oxidation or reduction in the electrolyte solution, the energy density available from batteries based on redox electrolyte solutions is limited generally by the maximum solubility of redox salts of the various oxidation states in the electrolyte solution, and in particular the redox component with the lowest solubility.
In the vanadium redox battery employing V(II)/V(III) and V(IV)/V(V) redox couples in the H2SO4 for the negative and positive ½-cell electrolyte solutions respectively, the vanadium concentration has been limited to less than 2M (about 1.8M) due to precipitation of V(II) and V(III) at low temperatures and the thermal precipitation of V(V) at high temperatures. The solubility of the V(II), V(III) and V(IV) ions increases with increasing temperatures, however, V(V) undergoes thermal precipitation to V2O5 at temperatures above 30° C.
For example if a 2M V(V) solution is exposed to temperatures of 30° C., a slight precipitate will start to form after 2 days, with heavy precipitation evident after only 4 days. At 40° C., a heavy precipitate will form after 2 days in a 2M V(V) solution. Even a 1.8M V(V) solution will precipitate after 6 days at 40° C.
This problem in use can be avoided by reducing the vanadium ion concentration to less than 1.8M for applications where the temperature is likely to exceed 40° C. and where the systems will be maintained in fully charged state for long periods. However in many applications it is not desirable to reduce the vanadium ion concentration below 2.0M since such a reduction effectively reduces the capacity and energy density of the battery.
In PCT/AU94/00711, a stabilised vanadium electrolyte solution was described which employed stabilising agents to inhibit the precipitation of vanadium from supersaturated solutions. Thus, 3M V(V) solution could be stabilised for several weeks by addition of 1–3 wt % glycerol, while 3M V(II) was stabilised by 1–3 wt % ammonium oxalate. A mixture of glycerol and ammonium oxalate inhibited precipitation of both V(II) and V(V) ions allowing a 3M vanadium electrolyte solution to operate successfully in a vanadium redox cell for close to six months. A large number of other organic and inorganic additives were also shown to inhibit the precipitation of vanadium from supersaturated solutions.
While these additives play a vital role in inhibiting precipitation of vanadium ions from supersaturated solutions of 2 to 4M vanadium, the author has found, surprisingly, that in the above case of V(V) solutions, at concentrations above 4M, the thermal precipitation reaction is completely inhibited even without the use of stabilising agents. Thus, a 5.5M V(V) solution produced by oxidation of 5.5M VOSO4 in 2M H2SO4 showed no signs of precipitation even after 6 weeks at 50° C.
One of the objects of this invention is thus an all-vanadium redox battery employing vanadium solutions of greater than 2M and especially above 1.8M, more typically above 2M, even more typically above 3M, 4M or 5M concentration which can operate over a wide range of temperatures and operating conditions. To avoid the precipitation of V(II), V(III) or V(IV) ions at these concentrations the operating temperature of the system is maintained above 25° C. However, it has also been discovered that with the use of suitable stabilizing agents, the operating temperature can be extended below 25° C.